Radar level gauges are in wide use for measuring the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby transmitted electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby transmitted electromagnetic signals are guided towards and into the product by a probe acting as a waveguide.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and receipt of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time and the propagation velocity along the probe of the electromagnetic signals.
Radar level gauges are often classified as either pulsed system or FMCW-systems. In pulsed systems, pulses are transmitted towards the surface of the product, and the distance to the surface is determined based on the time-of-flight of the pulse to the surface and back to the radar level gauge. In FMCW-systems, a signal with varying frequency is transmitted towards the surface and the distance is determined based on the frequency (and/or phase) difference between a transmitted signal and a simultaneously received signals. So-called pulsed FMCW-systems also exist.
Radar level gauges are in some cases used for applications where malfunction of the radar level gauge could result in dangerous situations.
For example, a radar level gauge with overfill prevention functionality must be extremely reliable.
Various measures are taken to ensure the reliability of radar level gauges, in particular radar level gauges with overfill prevention functionality, and to thereby reduce the risk of dangerous situations, such as overfill. For instance, radar level gauges and other important devices may be designed to fulfill a certain Safety Integrity Level (SIL) as defined by the international standard IEC/EN 61508. According to this standard, safety related systems may fulfill the requirements for different Safety Integrity Levels from SIL1 to SIL4, where SIL4 represents the highest safety integrity level and SIL1 represents the lowest safety integrity level.
The SIL-rating of a system is related to the probability of failure on demand, which is in turn a function of the failure rate and the time between proof tests. To maintain a certain SIL-rating, it is thus necessary to perform proof tests at a regular interval specified in accordance with the SIL-rating. For example, proof tests may need to be performed annually.
Proof tests are generally specified by the manufacturer of the SIL-rated system and it is the responsibility of the user of the system that the proof tests are carried out properly.
For radar level gauges with overfill prevention capability, many manufacturers specify that so-called two-point verification checks should be included in the proof test. In a two-point verification check, measurement of two different filling levels are performed and verified. Ideally, the two different filling levels should be far apart, i.e. one level close to the bottom of the tank and one level close to the top of the tank.
This procedure may take quite a lot of time and requires personnel to be involved in pumping and measuring. Furthermore, deliberately raising the level of product in the tank to a high level may actually involve an increased risk for a potentially dangerous overfill condition.